compat: backport sign_extend32()

compat: backport sign_extend32()

commit 5faec21f3a6973219481394534ce0e5f17507d84
Author: Andreas Herrmann <andreas.herrmann3@amd.com>
Date:   Mon Aug 30 19:04:01 2010 +0000

    bitops: Provide generic sign_extend32 function

    This patch moves code out from wireless drivers where two different
    functions are defined in three code locations for the same purpose and
    provides a common function to sign extend a 32-bit value.

    Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
    Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>